This invention relates to a process and apparatus for packing commodities such as foodstuffs into vacuum-sealed packs, and is particularly applicable to the smaller sized packs which are used to contain foodstuffs such as rashers of bacon or small joints of meat sold through retail outlets.
Vacuum-sealed packs containing foodstuffs for domestic consumption generally consist of the foodstuff enclosed between two superposed films of plastics material which are sealed hermetically together around the periphery of the pack with a margin between the seal and the foodstuff. This margin may be as wide as 3 or 4 cm in places. The packs are sealed under a vacuum of e.g. 4 Torr to ensure that the food is kept fresh.
In one method of packing domestic cuts of meats such as bacon, a continuous film of stretchable plastics material is first deformed, for example by heat and vacuum, to provide a series of pockets for receiving the foodstuff. The stretched pocketed film is then fed to a packing station where the slices of bacon are loaded in succession into the pockets, and a second continuous film is then applied on top of the first film. The first or lower film is wider than the second or upper film so that the lower film is left with uncovered margins along opposite longitudinal edges.
The superposed films are then indexed to a vacuum sealing station where each pocket is received in a heated die which can be raised to bring the films into engagement with an upper die. The webs are clamped transversely between the dies but are not at this stage permanently sealed together; the longitudinal edges of the dies engage the margins of the lower web only; due to its narrow width, the upper web is not at this stage gripped along its longitudinal edges.
The space within the closed dies is then evacuated through ducts at the outer margins of the dies, and the air is drawn from the pocket via the free outer longitudinal edges of the upper web. A heating plate is then lowered to heat the upper web and to heat seal the superposed edges of the films around the whole periphery of the pocket; at the same time, the lower film is heated through the lower die.
On heating, the material shrinks into contact with the product in the pack and the chamber is simultaneously aerated so that the material collapses closely around the product. The dies are then separated and the sealed pack is moved out of the vacuum chamber and is replaced by a succeeding pack for the next cycle of sealing operations.
In this method, which has hitherto been applied only to heat shrinkable films, where a transverse row of pockets are indexed into the sealing chamber in each step evacuation of the pockets in the central region of the web is slow and inefficient as the air must pass across the outer pockets before it is removed via the edges of the web. In addition, relative alignment of the upper and lower webs must be exact in order to enable the dies to engage the margins of the lower web without trapping the upper web and preventing exhaustion of the pockets, and this is difficult to achieve in practice.
The method and apparatus of the invention provides an alternative means for achieving a vacuum-packed commodity in which the risk of leakage is extremely low and which can be performed at high speed.